1. Field of the Invention
Embodiments of the present invention generally relate to directional drilling in a wellbore.
2. Description of the Related Art
Conventional directional drilling with jointed pipe is accomplished through use of a Bottom Hole Assembly (BHA) consisting of a bent housing directional drilling motor and directional Measurement While Drilling (MWD) tool in the following fashion.
To drill a curved wellbore section, the drillstring is held rotationally fixed at the surface and the drilling motor will drill a curved wellbore in the direction of the bend in its outer housing. This is termed “slide” drilling because the entire drillstring slides along the wellbore as drilling progresses. The wellbore trajectory is controlled by orienting the BHA in the desired direction by rotating the drillstring the appropriate amount at the surface.
To drill a straight wellbore section, the drillstring is rotated at the surface with the rotary table or top-drive mechanism at some nominal rate, typically 60 to 90 rpm. This is termed “rotating” drilling. In so doing, the tendency of the bent housing motor to drill in a particular direction is overridden by the superimposed drillstring rotation causing the drilling assembly to effectively drill straight ahead.
When drilling with coiled tubing neither “rotating” drilling nor rotational orientation of the BHA can be accomplished without the addition to the BHA of a special rotating device to orient the BHA since coiled tubing cannot be rotated at the surface in the wellbore. One such rotational device, or orienter, operates by rotating in even angular increments, for example 30°, each time the surface pumps are stopped and then re-started. After each pump cycle, the orienter locks into and maintains its rotational position. This “ratcheting” device allows the directional driller to position the directional assembly closely enough to the desired toolface orientation to allow the wellbore to be drilled in a particular direction.
One significant drawback to directional drilling with the ratcheting orienter described above is the fact that drilling must be stopped each time the orienter is actuated. For example, if a rotational change of 210° is needed, drilling is stopped, the BHA is lifted off-bottom, and the pumps must be cycled 7 times to rotate the BHA by the required amount. This non-productive time is significant and has an adverse affect on the average drilling rate. In the case in many Canadian wells, an entire well is drilled in a matter of 6 to 8 hours. The time spent orienting can become a significant portion of the total drilling time.
A second drawback to directional drilling with the ratcheting orienter relates to its inability to drill an effective straight wellbore section. As described above, in conventional directional drilling, continuous drillstring rotation is used to wash-out the directional tendency of a bent-housing motor. This produces a very straight trajectory. When drilling with coiled tubing and a ratcheting orienter, continuous rotation is not possible. Thus the driller is forced to orient slightly left of the desired path and drill some distance ahead. Then after stopping to re-orient right of the desired path, the driller drills ahead again. This process is repeated until the “straight” section is completed. The resulting left-right-left or “wig-wag” wellbore trajectory roughly approximates the desired straight path.
Therefore, there exists a need in the art for an orienter that may be used in a coiled tubing drillstring and that can switch between effectively straight drilling and curved drilling without halting drilling.